A high power load coupled to an input waveguide must satisfy several operational requirements. One requirement is the uniform dissipation of a large input power which is presented through the input waveguide as a narrow and high energy density microwave beam. A second requirement is the reflection and distribution of input power in a manner which minimizes the formation of standing waves in the load, since standing waves can result in electric field enhancement and plasma arcing, which causes non-sustainable erosion of the load device. A third requirement is the minimization of reflected energy back to the input port.
Prior art microwave loads have attempted to trade off some of these requirements against other requirements. A prior art device capable of handling high input power density is described in U.S. Pat. No. 5,949,298 by Ives et al. In the device of Ives, RF power travels from an input waveguide into a cylindrical cavity to a far wall reflector, and the reflected power is subsequently directed against a plurality of dissipation surfaces. One difficulty of this prior art device is that some fraction of the input energy is reflected back to the input port. A computed and observed reflected power coupling of the prior art device of Ives shows 6% or more (−12 dB) of the applied power is reflected back to the input port. Because the input port of this device is exposed to a fraction of the reflected power in the cylindrical dissipation cavity, it is not possible to reduce the reflected input power below this level. A new microwave load device is desired which provides an additional reduction in the level of power reflected back to the input port. Additionally, the device of Ives is input power limited by the power density presented to the first reflection surface from the rotating reflector for certain traveling wave modes. For example, HE11 mode waves have a radial Gaussian energy profile with a “hot spot” at the center of the microwave beam which impinges on the coated interior wall, and removing heat from this beam profile with an elevated central power density limits the power handling capacity of the entire device, since power density of the central beam hot spot governs the temperature rise of the RF absorbing coating 140, and an RF absorptive coating such as black rutile is limited in operating temperature to less than 300° C. before damage to the coating occurs.